Pressure-sensitive adhesive sheet



Patented :July 20, 1948 PRESSURE-SENSITIVE ADHESIVE sneer Ellington M.Beavers.

to The Resinous pany, Philadelphia, ware Pro Philadelphia, Pa., assignorducts & Chemical Com- Pa.,-a corporation of Dela- No Drawing.Application November 30, 1945,

Serial No. 632,115 3 Claims. (01. 117-422) This invention relates to apressure-sensitive adhesive sheet or tape compris ng a flexible backingcoated with a layer of adhesive which is tacky on the surface and issecurely bonded to the backing.

An object of this invention is to provide a pressure-sensitive adhesivesheet which is extremely durable, the adhesive of which is not subjectto cold flow and is resistant to organic solvents, particularlyhydrocarbon solvents. A further object is to provide adhesive sheetswith adhesives of varying de rees of surface tackiness.

These objects are atta ned by coating a flexible backing material with alayer of an unsaturated, non-crystalline, curable, tacky, thermoplasticlinear polyester which is capable of crosslinking and which has admixedtherewith an organ c peroxidic catalyst, and thereafter curing saidlayer at a temperature above 100 C. while the surface of sa d layer isexposed to oxygen.

By pressure-sensitive adhesive sheet is meant an article consisting of aflexible backing coated with an adhesive layer, the surface of which istacky and does not require treatment with water, solvent, heat, or otheragents in order to impart adhesiveness.

Pressure-sensitive adhesive sheets and tapes known heretofore havesuifered from several disadvantages which it is the object of thisinvention to overcome. For example, the adhesives used in makingadhesive sheets and tapes heretofore were usually combinations ofseveral ingredients such as rubber, thermoplastic resins, plasticizers,and the like. As a result, the viscosity, cohesiveness, and adhesivenessof such tapes varied Widely with changes in temperature and on aging.The adhesive often became brittle at winter temperatures and/orobjectionably sticky at summer temperatures. In many instances, theviscosity and the adhesiveness of the layer of adhesive changed on agingor at elevated temperatures with' the result that the rolls of tapebecame deformed and the layers stuck together with such tenacity thattheycould not be separated without rupturing the adhesive layer, orbacking, or both.

These and other disadvantages are overcome by this invention; Flexiblebacking materials are'first coated with films or layers of a mixture ofan unsaturated, non-crystalline, tacky, thermoplastic linear polyestercapable of cross-linking and a peroxidic catalyst. The him is then curedin the presence of oxygen. The polyester is thus converted to an.infusible and insoluble layer which adheres very tenaciously to thebacking and. at the same time, has a tacky surface. By curing the layerin the presence of oxygen, the curing of the surface of the polyester isinhibited; and, while the layer remains tacky, it is cured sufficientlyto insure its having the necessary cohesive strength to preventsmearing.

2 The remainder of the film is converted to the insoluble and infusiblecondition. Thus, there is obtained a tape which is tacky on the surface,as pressure-sensitive adhesives must be, but is extremely durable,stable, and resistant to solvents because the remainder of the adhesivelayer is in the cured condition.

The l near polyesters which are employed in making such a desirableproduct are those prepared by the esterification of 1,2-propyleneglycol, a saturated unsubstituted aliphatic dicarboxylic acid such assebacic acid, and an unsaturated dicarboxylic ac d such as maleic. Suchpolyesters are essentially of the linear or two-d mensional type and arepartially unsaturated due to the presence of the double bond in theresidue of the esterified unsaturated acid. These double bonds impart tothe polyesters the ability to cross-link, under the influence of heatand/ or a catalyst, and to form thereby three-dimensional polymers whichare no longer thermoplastic. The convers on of the two-dimensionallinear polymers to t e three-dimensional polymers is commonly known ascuring.

The polyesters which are employed herein are linear polyesters ofrelatively high molecular weight containing alternate chains fromdicarboxylic acids and 1,2-propy1ene glycol. the chains from thedicarboxylic acids in the linear polyesters consisting of a majorproportion of chains from a saturated dicarboxylic acid having a chainlength of at least four carbon atoms and carboxyl groups as the solereactive functional groups and a m nor proportion of chains from anair-unsaturated dicarboxylic acid.

The polyester may be made by well-known methods. The preferred processof manufacture comprises heating a saturated dicarboxylic acid, anunsaturated dicarboxylic acid, and an excess of a dihydric alcohol inthe presence of an acid catalyst such as zinc chloride. In the firststages of the reaction, esterification takes place with the eliminationof water. Thereafter, condensation occurs withthe splitting out andremoval of the excess glycol. In this way, polyesters having highmolecular -'weights may be prepared in 'a minimuinoftime.

The saturated dicarboxylic acids which are preferred are those aliphaticacids having at least two carbon atoms between the carboxyl groups or atotal of at least four carbon atoms including the carbonatoms of thecarboxyl groups. Furthermore,. the dicarboxylic acids of first choice donot contain any groups, other than the carboxyl groups, whichvarereactive under the conditions employed in the formation of the linearpolyesters. They may thus be described as unsubstituted, and the word"unsubstituted is used in that sense here and elsewhere throughout thisspecification. Thus, succinic, adipic, amethyl glutaric, .pimelic,suberic, azaleic, sebacic,

oss caf 1 J v and higher acids may be used. Sebacic acid is particularlysatisfactory and is preferred. Mixtures of saturated acids, such as amixture of sebacic and adipic acids, may also be employed.

Suitable a,fi-unsaturated dicarboxylic acids are those which areunsubstituted and which combine, as do the saturated dicarboxylic acids,to

form linear polyesters made up of alternating residues of thedicarboxylic acids and dihydric alcohols. Included as operable aremaleic, fumaric, citraconic, and mesaconic acids. Maleic acid is muchpreferred, and it may be used in the form of its anhydride or a loweraliphatic ester which reacts by transesterification. The amount ofunsaturated acid which is used has a marked eflect upon the propertiesof the polyester. As indicated above, the presence of the double bondsin the residue of the esterified unsaturated acid imparts to the resinthe capacity for forming cross-linkages, resulting in conversion tothreedimensional .polymers. It is convenient and customary to expressthe amount of unsaturated acid as a molar percentage of the total acidspresent, and'this system has been adopted herein. The amount ofunsaturated acid employed will depend largely upon the properties to beattained in the finished polyester and upon the conditions of operation.In a general way, the maximum molecular weight obtainable in anessentially linear polyester, other factors being constant, will varyinversely with the amount of unsaturated acid employed. In cases where aresin of extremely high molecular weight (for example, 100,000) isprepared, the amount of unsaturated acid employed is very low and may beas low as 0.1% of the total acids used. How ever, the amount ordinarilyused is above 1%. The maximum amount is about 20%. For mostapplications, an amount between about 1% and about 6% is much preferredbecause this proportion of unsaturated acid assures a sufllciently highdegree of unsaturation in the polyester to permit cross-linking and alsopermits the esterification to proceed until the product has a sumcientlyhigh molecular weight.

The molecular weight of the polyester, which is the essential ingredientof the adhesive used herein, may be conveniently estimated fromviscosity measurements and the use of Staudingers formula as given inhis book Die Hochmolekularen Organischen Verbindungen (1932 Berlin). Thepolyesters employed in this invention may have molecular weights as lowas 5000 and as high as about 100,000, but those having a molecularweight from about 8000 to about 20,000 are preferred because they may beused without further modification with hardening or plasticizing agents.

The catalysts which may be used are organic peroxidic compounds such asbenzoyl peroxide, acetyl :peroxide, acetone peroxide, tert.-.butylhydroperoxide, di-tert,-butyl peroxide, the barium salt of tert.-butylhydroperoxide, di-tert.- butyl diperphthalate, tert.-butyl perbonzoate,and the like. Definite preference is given to those catalysts whichcontain a tertiary butyl group attached to a peroxidic oxygen atombecause the adhesive sheets so prepared have better agingcharacteristics than those catalyzed with acyl peroxides. The amounts ofthese catalysts which are used are from about 0.5% to about dependingupon the particular catalyst and the degree of unsaturation of theparticular polyester. Ordinarily, an amount of catalyst from about 1% toabout 3% is preierred.

The mixture of curable, tacky linear polyester may be applied to thebacking by such methods as roller-coating, spraying, knifing, and thelike. The mixture may be applied in the undiluted form, in which case itis conveniently warmed in order to reduce its viscosity. Or it may beapplied in the form of a solution in organic solvents, in which case thesolvent is allowed to evaporate or is driven off during the curing step.Suitable solvents comprise hydrocarbons such as benzol, xylol, toluol,hydro-aromatic solvents such as "Solvesso #2, ketones such as acetone,esters such as ethyl acetate, and chlorinated hydrocarbons such asethylene dichloride. Hydrocarbons are preferred. One advantage in usingthis type of adhesive is that very highly concentrated solutions may beapplied, and a relatively thick layer of adhesive can ,be laid down in asingle operation, in contrast to other adhesives containing rubber,which can be applied only in the form of relatively dilute solutions.

The backing materials may vary' widely from flexible, impervious metalfoils to thin, porous sheets of paper. Among the suitable backingmaterials are textiles of varying degrees of thickness and perviousness,including glass cloth, sheets of regenerated cellulose typified byCellophane, paper such as kraft, wrapping, crepe, tissues, and the like.One of the advantages of this type of adhesive is that it can be appliedin the form of a solution which, even at high concentrations, canthoroughly impregnate a pervious backing, if desired.

During maintained in an atmosphere containing oxygen. In most cases, itis suflicient to conduct the heating in air, although higherconcentrations of oxygen are recommended when adhesives which cure veryrapidly are employed.

The following examples are illustrative of this invention:

A. Into a three-necked flask equipped with a mechanical stirrer,thermometer, and reflux condenser were charged 354 grams of sebacicacid, 43 grams of maleic anhydride, 208 grams of 1,2- propylene glycol,and 0.4 gram of zinc chloride. The mixture was stirred and heated to 200C., at which point is was held for six hours. During this period, thepressure was gradually lowered to 50 mm. of mercury. The condenser wasreplaced with a simple gooseneck, and the pressure was reduced to 5 mm.After being heated at 200 C. under a 5 mm. pressure for four hours, thebatch was cooled.

The resulting polyester had an acid number of 1.6 and a viscosity, whenmeasured as a 50% solution in ethylen dichloride, of U on theGardner-Holdt scale.

B. A 50% solution of the resulting polyester was prepared inahydrocarbon solvent known as "Solvesso #2. To this solution was added 2%of benzoyl peroxide, calculated on the weight of polyester in solution.This solution was flowed out on strips of Cellophane and kraft paper bymeans of a Film-O-Graph, which laid down a wet film 25 mils inthickness. The strips were placed in an oven at C. for twenty minutes.The oven was equipped with a fan, which drew in and circulated air overthe surfaces of the coated strips, The strips were then removed from theoven and cooled. The coated strips were found to be tacky on the surfacebut otherwise well cured. The adhesion of the polyester to the backingmaterials was extremely tenacious and, particularly in the case of thekrait paper strips, it

the heating, the layer of adhesive is was evident that the paper hadbeen thoroughly impregnated.

C. The strips were-applied by pressure of the hand to various surfaces,including steel, wool, glass, Holland cloth, and Plexiglas, the lastbeing a sheet of an acrylic plastic. In all cases, the tape could bestripped off cleanly without leaving the slightest smudge.

Strips were repeatedly affixed and removed from a sheet of glass, and itwas found that this could be repeated at least twelve times without theslightest evidence of smearing of the glass or a lowering of adhesion.Other strips were aflixed to glass sheets and placed in an oven at 45 C.for twenty hours. .At the end of this time, the tape stripped cleanlyfrom the glass.

D. Tapes were prepared by coating strips of aluminum foil and glasscloth (Fiberglas ECG-11462) in the manner described in B above. Thesetapes, like those in C above, had good adhesion to many differentsurfaces.

E. Tapes were prepared by coating strips of. Cellophane in the mannerdescribed in B above. These were immersed in gasoline (StandardReference Fuel #6) for twenty-four hours, together with strips ofcommercial Cellophane tape as standards. The commercial tape swelled toa considerably greater extent than did the tapes of this invention; and,when the tapes were dried in air, the products of this invention werefound to have much better adhesion than the commercial tape.

Strips of the two tapes were pressed on glass, and the pieces of glasswere immersed in gasoline for twenty-four hours. On removal of thepieces of glass from the gasoline, the tapes were stripped therefrom.The tape made as described above stripped cleanly from the glass,whereas the commercial tape left a film or smear of adhesive on theglass.

It is apparent from the above examples that unusual pressure-sensitiveadhesive sheets or tapes result from this invention. Not only is thesurface of the adhesive sheet sufliciently tacky to insure adequateadhesion to dry surfaces, but the cohesive strength of the curedpolyester is so high as to prevent splitting or "picking of the adhesivelayer. Furthermore, the adhesion of the adhesive layer is so great, dueto the curing of the film in contact with the backing, that there is noseparation of adhesive and backing. The adhesive layer is resistant tosolvents which cause swelling of many adhesives formerly employed. Inaddition, the adhesive layer does not soften at summer temperatures anddoes not flow under such conditions. Durability depends in part upon thechoice of curing catalyst, and the most durable products are thosecatalyzed by the group II metalsalts of tertiary butyl hydroperoxide. ofwhich the calcium, barium, and strontium salts are typical.

While all of the sheets or tapes 01', this invention are characterizedby the above properties. they may be varied slightly in such propertiesas toughness and tackiness by changing the ratios of ingredients'and thecuring treatment. Thus. by varying the amount of catalyst, thetemperature of cure, and the concentration of oxygen, there may beprepared adhesive sheets varying in tackiness from those described asvery sticky to those having only a very slight tack. Also,

they may be modified with such materials as.

mineral oil, high-boiling ester plasticizers, rosin, extenders, andpigments. Modification is often desirable when the the preferred range Iclaim: v

1. A pressure-sensitive adhesive sheet, which is flexible and willconform to a surface against which it is pressed, which comprises aflexible backing to which is securely anchored an insolubleresinouscoating having a tacky surface and the remainder in an infusible state,said resinous coating being the product resulting from heating at atemperatureof 100 C. to 225 C. in contact with said backing and withoxygen a mixture of 0.5 to five ing agent and one hundred parts of atacky, non-crystalline, linear polyester having a molecof 8000 to20,000.

ular weight from about 8000 to 20,000 and containing alternate chainsfrom dicarboxylic acids and 1,2-propylene glycol, the chains of thedicarboxylic acids in the linear polyester consisting of to 99% ofchains from sebacic acid and 2.0% to 1% of chains from maleic acid.

' 2, A pressure-sensitive adhesive sheet, which is flexible and willconform to a surface against which it is pressed, which comprises aflexible backing to which is securely anchored an insoluble resinouscoating having a tacky surface and the remainder in an infusible state,said resinous coating being the product resulting from heating at atemperature of C. to 225 C. in contact with said backing and with oxygena mixture of one to three parts of curing agent and one hundred parts ofa tacky, non-crystalline, linear polyester having a molecular weightfrom about 8000 to 20,000 and containing alternate chains fromdicarboxylic acids and 1,2-propylene glycol, the chains of the dicarboxylic acids in the linear polyester consisting of 94% to 99% ofchains from sebacic acid and 6% to 1% of chains from maleic acid.

3. A pressure-sensitive adhesive sheet, which is flexible and willconform to a surface against which it is pressed, which comprises aflexible backing to which is securely anchored an insoluble resinouscoating having a tacky surface and the remainder in an infusible state.said resinous coating being the product resulting from heating at atemperature of 100 C. to 225 C. in contact with said backing and withoxygen a mixture of 0.5 to flve parts of a peroxidic curing agent andone hundred parts of -a tacky, non-crystalline, linear polyester havinga molecular weight from about 5000 to 100,000 and containing alternatechains from dicarboxylic acids and 1,2-propylene glycol, the chains ofthe dicarboxylic acids in the linear polyester consisting of 80% to 99%of chains from sebacic acid and 20% to 1% of chains from maleic acid.

- ELLINGTON M. BEAVERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Ind. 8: Eng. Chem," of November1937, pages 1267-4269.

molecular weight is outsideparts of a peroxidic cur-

